It's not that they were poor quality or poorly made
It's because after at least 60 years, with sweat and oil from your hand, the leather becomes brittle and the cotton stitching rots.
Modern replacements are probably the same quality but have the advantage of being new. In 20-30 yrs time they may well have deteriorated too, perhaps even sooner if they get used as often as many would have originally in the 30's, 40's and 50's.
It's not what you'd call a good and practical design though, and probably why it wasn't used on regular luggage, , but you can see it's appeal to manufacturers being cheap and easy to make, and being flat against the cabinet would have made it safer and easier to ship in a cardboard box.